Guide to Randomization on WTH W2H
While there are customizable options for randomization on WTHW2H, certain options are standard and other options may require additional development and associated costs.
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- All types of randomization procedures can have weights assigned to each arm.
- By default, each arm has a weight of 1. Since all arms have the same weight the arm weights are irrelevant and randomization will happen as expected for the selected randomization option.
- Once an arm has been assigned a weight, all other arms will default to a weight of 0 unless manually assigned. If an arm has a weight of 0 no participants will be randomized in to that arm.
- Arm weights are useful only when you must have arms of different sizes.
- Example: a two arm, 150 participant study consisting of a control arm and an incentive arm
- If we want a larger control group than incentive group, we could set the following arm weights
- Control arm: 2
- Incentive arm: 1
- Since the control arm has twice the weight of the incentive arm participants are twice as likely to end up in that arm than the incentive arm. In order to maintain proper randomization we would need the control arm to be twice as large as the incentive arm.
- If this study were to be blocked, the block sizes would have to be a multiple of 3 (control weight 2 + incentive weight 1). A block of size three would have 2 control participants and 1 incentive participant. A block of size 9 would have 6 control participants and 3 incentive participants.
- If we want a larger control group than incentive group, we could set the following arm weights
Understanding Caps:
- In WTH W2H you will have to select the size of each of your arms. However, it is important to set the cap of each arm a bit higher than the target, so that study staff will continue to remain blinded to which arm each participant is being randomized as full enrollment is approaching and up until the last participant is enrolled. Close attention should be paid to when you are approaching full enrollment so that enrollment can be ceased as soon as you’ve reached your enrollment goal.
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- The most effective way to ensure randomization is working as you would expect is to enroll a reasonable number of participants and observe what arms they are assigned to. Ensure that the proportions assigned to each arm are as expected, and if using blocking, verify that the desired block sizes are appearing.
WTH W2H role permissions and access to randomization details:
- WTH W2H Admins can see the current state of the blocks.
- WTH W2H Coordinators can’t see current state of the blocks so they will remain blinded to how the blocks are being filled
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